industrial large area n-type solar cells with ... - ISC Konstanz

INDUSTRIAL LARGE AREA N-TYPE SOLAR CELLS WITH ALUMINIUM REAR
EMITTER WITH STABLE EFFICIENCIES
R. Kopecek 1 , A. Halm 1 , L. Popescu 1 , K. Peter 1 , M. A. Vázquez 2
1 International Solar Energy Research Center - ISC Konstanz, Rudolf-Diesel Str.15 , D-78467 Konstanz, Germany
e-mail: radovan.kopecek@isc-konstanz.de
2 Isofotón, S.A., c/ Severo Ochoa 50, Parque Tecnológico de Andalucía, 29590 Málaga, Spain
ABSTRACT
We present in average 17% efficient large area
(156x156mm 2 ) n-type boron-free Cz-Si solar cells with
rear Al emitter fabricated in an industrial like environment
(average V oc=627.7mV, J sc=34.6mA/cm 2 , FF=78.1%). The
processing sequences were intentionally kept very close
to the conventional p-type firing through SiNx process due
to a simple transfer to existing p-type production lines.
Single processing steps were optimised or slightly adapted
to the n-type structure such as FSF formation, edge
isolation, paste selection, firing conditions and rear side
geometry for cell interconnection. The main advantage of
such solar cell structure is that the device is completely
boron free and therefore stable in efficiency when exposed
to light and in addition show an excellent performance at
low light intensity.
We show that, if we improve the front side properties such
as improved FSF (e.g. selective FSF) and adapted
passivation, stable efficiencies around 18% are possible
with our simple technology. To go beyond 18% more
advanced process on the front is needed such as fine-line
printing or even elimination of the front contacts to the
back resulting in an IBC cell with local Al or B-emitter. With
the screen printing process an efficiency of around 19.5%
should be feasible.
One important part of this work was to identify and test
different interconnection schemes of cells with the focus
on the rear side of the cell. We present simple methods
how shunting of the standard rear side has been
overcome and therefore the interconnection of strings can
be done in an even more simple than conventional way.
In the past a few groups, including ours, started working
on n-type Al rear emitter solar cell development with
respectable results [2, 3, 4, 5]. However most of them
presented their results on cell level without the proposal
on how to interconnect the cells within a module. We
present here a very simple process resulting in cells with
stable efficiencies above 17% with the possible
implementation of AgAl pads on the rear side, or even
without them. The relevance is high as the solar cells
could be processed in existing production lines and would
show no degradation in their performance.
2. EXPERIMENTAL
Solar cell process description
Figure 1 shows the process flow chart for our n-type Cz-Si
solar cells. It starts with an alkaline texture followed by a
P-diffusion in an open tube furnace. Then the edges are
wet chemically isolated in an inline bench. After removal of
PSG a PECVD SiNx is deposited on the front side, front
and rear paste is screen printed, dried and co-fired in beltfurnace.
1. INTRODUCTION
Today’s standard efficiencies of industrial Cz-Si solar cells
based on firing through SiN x process reside between 17%
and 18%. More advanced structures using selective
emitters or an improved rear side lead often to efficiencies
above 18%. However, most cell concepts on mono c-Si
are still adapted on p-type Cz-Si material, resulting
(particularly in the case of this type of material) in a large
light induced degradation of often up to 1% absolute of the
initial efficiency due to the formation of BO 2i complexes. In
addition, the performance at low light intensities is
predicted to be better for n-type solar cells as the lifetime
increases in contrary to p-type cells [1], which would lead
to a much better power output averaged over the year.
Figure 1: Al rear emitter solar cell: top view (left) and
process flow chart (right).
To implement the AgAl pads, areas on the rear side under
the front busbars are opened by abrasion of the fully
printed Al. We have shown in previous publication that this
can be done quickly and without any damage to the rear
side emitter [6]. In the last section of the result we will
show our latest results on the novel interconnection
technology using our abrasion method in the combination
with a new product applied in PV industry from Hitachi
Chemical which leads to excellent mechanical properties.

3. RESULTS
Solar cell process optimisation
In order to optimise the solar cell performance we have
varied individual cell processing steps, especially the rear
side morphology. Figure 2 shows the experimental flow.
The alkaline texture and chemical edge isolation was
optimised already in previous experiments. Here we have
chosen e.g. a textured versus a polished surface .
group G1 G2 G3 G4
number of wafers 16 16 5 5
base type n n p p
alkaline texture
single side polish
FSF 65 Ohm/sq
wet chemical edge isolation
PECVD SiNx
screen printing
fast Firing
Figure 2: Experimental process flow chart on n- and p-
type solar cells.
Table 1 depicts the average solar cell parameters for all 4
groups.
Table 1: Solar cell results from different groups
explained in figure 2.
group G1 G2 G2 G4
η [%] 16.9 17.0 17.4 17.4
V oc [mV] 627.6 627.7 627.9 628.5
J sc [mA/cm 2 ] 34.5 34.6 36.0 36.3
FF [&] 78.1 78.3 76.9 76.3
Average cell efficiency was taken (5 cells for p-type and
16 cells for n-type) from each group. It is visible that the p-
type solar cells are better performing because the n-type
concept is very sensitive to the material quality. In addition
it can be seen that polishing of the rear side does not have
any additional advantage as the formed Al emitter is
completely dissolving and re-crystallising 7-10µm of the
rear surface. The average efficiencies for the better n-tyoe
cell group are 17.0% with a best cell of 17.2%.
PC1D simulations show that further optimisation of front
side properties like emitter with higher sheet resistance
FSF with the combination of improved SiN x passivation will
further increase the efficiency to values close to 18%. This
will be explained in the following.
Figure 3: PC1D simulations of efficiency as a function
of front surface recombination velocity for Al-rear
emitter solar cells.
When reducing the SRV by a factor of 10 (improved FSF
and passivation) efficiencies of 17.8% are reachable and
by a factor of 100 even 18% are possible. We have
already tested this enhancement on p-type cells using
selective P-emitter and therefore we are optimistic to
transfer it to our n-type structure as well. An additional
improvement is foreseen when the shadowing is reduced
or even completely eliminated (IBC Al or B-rear junction).
Then efficiencies of around 19.5% are possible with
screen printed contacts.
Concepts for solar cell interconnection
One of our solution of implementing rear tabbings without
causing tremendous shunts is that the AgAl paste is
printed on the emitter which can be done in several ways.
We propose an easy process which is demonstrated in
Figure 4. After the fully printed Al rear contact is fired and
emitter formed the stripes for AgAl pads are mechanically
removed, filled with AgAl paste by screen printing and
fired again. We have shown that this mechanical abrasion
is not causing any surface damage in our last
publication [6].
Further potential of improvement
Figure 3 shows a PC1D simulation of the reachable
efficiency as a function of surface recombination velocity
assuming a FF of 78.5% and a metallization shadowing of
8%. Currently we have reached average efficiencies of
17.0% with a SRV 5x10 5 cm/s.
Figure 4: Schematic drawing of the mechanical
abrasion of the stripe areas for AgAl pads.

Other possibility would be to print or contact directly on the
Al paste which actually today also works with several
techniques such as ultrasonic soldering or bonding. The
attached contact might stick perfectly to the Al, however
the problem is that the Al does not hold on the Si and the
contact can be easily pealed off as already showed by us
at the Hawaii conference in 2006 [2].
Figure 5: Schematic drawing of the rear side with
different interconnection options: a) standard p-type
AgAl pad geometry, b) selective opened Al-areas after
fully sided printed fired rear side c) contacting of
selective opening and d) direct contacting of Al.
Figure 5 shows different possibilities of the rear side of an
n-type rear Al-emitter solar cell. Picture 5 a) is the
classical implementation the AgAl pads. However for our
n-type cells this is not possible as we would contact the
base directly with the emitter contacts. Therefore we have
proposed to fully print the Al rear side in the past which
results in a full Al emitter layer. After abrasion of the rear
stripes behind the front bus-bars (picture 5 b)) a low firing
AgAl paste printed and fired as depicted in picture 5c).
The last picture 5d) shows a AgAl paste directly printed on
the Al which gives a good adhesion but gets pealed off on
the interface SiAl eutectic-Al contact.
Our method works perfectly but includes one more firing
step and an additional abrasion step. Therefore we have
looked for other possibilities. Together with Hitachi
Chemical we have developed an interconnection concept
based on their existing product conductive film CF and our
selective abrasion concept.
Product CF was developed for the electronic industry and
can be used for bonding of tabbings on different surfaces
such as Ag, AgAl, Al and Si. Figure 6 shows such tabbing
experiments on a) AgAl-paste, b) SiAl eutectic, c) Al with
no Si edge and c) Al with Si edge.
The adhesion of all samples is excellent, however for
sample c) the tabbing is pealing off because of the already
explained effect. However if, as in standard cells, a
shallow Si-edge is left, the tabbing is attached to that and
avoids the peal-off. This brought us to the idea co combine
the use of CF with our abrasion method in a selective way
as depicted in figure 7.
Figure 7: Schematic drawing of a ) fully metallised
rear Al-emitter solar cell b) selectively abrased rear
side and c) tabbed rear side on the selective
openings.
The rear side process will then be extremely simple
resulting in three steps. Fully screen printing of Al rear
side and firing; selective abrasion of the Al rear contact;
bonding of tabbings. The opened regions guarantee a
good adhesion to the emitter and the Al regions a good
electric contact to the entire fully metallised rear side.
Now we even have several advantages compared to the
standard p-type side such as:
- elimination of one printing step
- saving of AgAl paste
- low temperature bonging process (lower stress)
Figure 6: Schematic cross sections (left) and top view
pictures (right) of different interconnection geometries
of the tabbing.
This method can be of course used for p-type rear sides
as well leading most likely even to slightly higher Vocs
because of the fully developed BSF.
This method is currently under intensive testing and the
results will be presented at the EU PVSEC 2010 in
Valencia. At hat time mechanical as well as electrical
properties will be evaluated in detail. In addition
degradation tests as well as cost of owner ship
calculations will be conducted in order to see the full
potential of such contacting technology.

4. CONCLUSIONS
We present a simple solar cell concept for the processing
of n-type rear Al emitter solar cells based on the p-type
technology leading to stable efficiencies of 17%. Additional
improvement of the rear side FSF and passivation would
lead to efficiencies of around 18%.
We have introduced a simple and effective interconnection
method for the rear side of the cell as this side behaves
differently from the p-type process. This interconnection
method includes one additional step of Al abrasion but
gets rid of the printing of AgAl pads for soldering.
In our next experiments solar cells will be processed with
this method. IV measurements as well as the long term
stability testing will be performed on cell as well as on
module level.
[6] A. Halm, L.M. Popescu , J. Theobald , K. Peter , R.
Kopecek, LOW TEMPERATURE PADS ON Al-EMITTER
OR Al-BSF, 24rd European Photovoltaic Solar Energy
Conference and Exhibition Hamburg, Germany (2009)
ACKNOWLEDGEMENT
Most of this work was done within the AlFil project funded
by the Spanish Government and Isofotón.
The authors would like to acknowledge the excellent
collaboration with S. Takeda, M. Kawakami, T. Hattori and
others from Hitachi Chemical and P. Motyka from Nissin
on the interconnection work.
REFERENCES
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World Conference on Photovoltaic Energy Conversion,
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